The membranes of erythrocytes invaded with certain malaria parasites (plasmodium falciparum) contain knoblike structures which cause the cells to be trapped in capillaries, resulting in high morbidity and mortality rates. Little is known about the composition of the "knobs" which are about 100 mm. in size. Analytical electron microscopy is being applied to thin sectons of monkey erythrocytes invaded with malaria to search for iron in possible concentrations of heme, and nitrogen in regions containing possible concentrations of histidine. It is planned to label histidine with fluorine to localize this compound in the cells.